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Thermodynamic Properties of Three Pyridine Carboxylic Acid Methyl Ester Isomers

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Thermodynamic Properties of Three Pyridine Carboxylic Acid Methyl Ester Isomers 580 J. Chem. Eng. Data 2007, 52, 580-585 Thermodynamic Properties of Three Pyridine Carboxylic Acid Methyl Ester Isomers Maria D. M. C. Ribeiro da Silva,* Vera L. S. Freitas, Luı´s M. N. B. F. Santos, Michal Fulem, M. J. Sottomayor, Manuel J. S. Monte, and W. E. Acree, Jr. Centro de Investigac¸a˜o em Quı´mica, Departamento de Quı´mica, Faculdade de Cieˆncias, Universidade do Porto, Rua do Campo Alegre, 687, P-4169-007 Porto, Portugal The present work reports the values of the standard (p° ) 0.1 MPa) molar energies of combustion for the liquid isomers methyl picolinate and methyl isonicotinate and for the crystalline isomer methyl nicotinate measured by static bomb calorimetry. The molar enthalpies of vaporization of the three isomers at T ) 298.15 K as well as the molar enthalpies of sublimation of two of them (methyl picolinate and methyl nicotinate) were derived from vapor pressure measurements at different temperatures using a static method. The molar enthalpies of vaporization at T ) 298.15 K of methyl picolinate and methyl isonicotinate were also measured using Calvet microcalorimetry. The thermal behavior of the compounds was studied by differential scanning calorimetry, and the temperatures and the molar enthalpies of fusion were determined using this technique. The experimental results were used to derive the gaseous standard molar enthalpies of formation, at 298.15 K, of the three pyridinecarboxylic acid methyl ester isomers and the triple points of the ortho and meta isomers. Introduction Thermochemical data are essential for the understanding of the relations between the structure and reactivity of molecules, and that reason justifies the importance of the study of key compounds from several classes of compounds because such data permit clarification of the chemical behavior of the corresponding species. In this context, systematic experimental and theoretical thermochemical studies on heterocyclic mol- Figure 1. Molecular formulas for pyridinecarboxylic acid methyl ester ecules derived from pyridine have been done in our Thermo- isomers: (a) methyl picolinate, (b) methyl nicotinate, and (c) methyl chemistry Laboratory for more than a decade in order to evaluate isonicotinate the energetic effects of different substituents on the pyridinic ring.1-14 The main goal of the present work is to determine the gaseous enthalpies of formation of methyl picolinate (2- 3-PyCOOCH3 (CASRN 93-60-7) and 4-PyCOOCH3 (CASRN PyCOOCH3), methyl nicotinate (3-PyCOOCH3), and methyl 2459-09-8) were prepared by the esterification of nicotinic acid isonicotinate (4-PyCOOCH3), represented in Figure 1, together and isonicotinic acid, respectively, with trimethylorthoacetate with the measurement of other relevant properties to the according to the published synthetic method of Trujillo and thermodynamic characterization of those isomers. Gopalan.15 Both products were collected and further purified The present work reports for the three pyridinecarboxylic acid by fractional distillation under reduced pressure. The purities methyl ester isomers the experimental values of the standard of 2-PyCOOCH3 and 4-PyCOOCH3 were checked by GLC molar energies of combustion, the standard molar enthalpies of (using an HP 4890 apparatus with a 15 m length column HP-5, vaporization, and the standard molar enthalpies of sublimation 5 % diphenyl, and 95 % dimethylpolysiloxane under a nitrogen s of two of them methyl picolinate and methyl nicotinate. For pressure of 21 kPa; the temperature oven was 423 K), whereas methyl isonicotinate, the enthalpy of sublimation was estimated the purity of 3-PyCOOCH3 was confirmed by DSC after it has from the enthalpies of fusion and of vaporization. been dried at T ) 303 K under reduced pressure. For The experimental results are used to derive the gaseous 3-PyCOOCH and for 4-PyCOOCH , microanalytical determi- ) 3 3 standard molar enthalpies of formation, at T 298.15 K, of nations yielded the following results (mass fractions). For the three pyridinecarboxylic acid methyl ester isomers. In order 3-PyCOOCH3:C7H7NO2, C, 0.6139; H, 0.0516; N, 0.1011. For to complement the results, the thermal behavior of the com- 4-PyCOOCH :CH NO , C, 0.6137; H, 0.0507; N, 0.1013. pounds was studied, and the temperatures and the molar 3 7 7 2 Calcd C, 0.6131; H, 0.0514; N, 0.1021. The percentage of CO enthalpies of fusion were determined by differential scanning 2 recovered in the combustion experiments also showed the high calorimetry. purity of the samples. Experimental Section The densities (F) of the samples and the average ratios (µ) Materials. The compound 2-PyCOOCH3 (CASRN 2459-07- of the mass of carbon dioxide recovered to that calculated from 6) was a commercial sample from Aldrich, 99 %. The isomers the mass of sample, together with the standard deviation of the F) ‚ -3 16 * Corresponding author. E-mail: [email protected]. Phone: +351 22 6082 mean, were as follows: 2-PyCOOCH3, 1.137 g cm , µ -3 838. Fax: +351 22 6082 822. ) (0.9999 ( 0.0003); 3-PyCOOCH3, F)1,0 g‚cm (assumed), 10.1021/je060472e CCC: $37.00 © 2007 American Chemical Society Published on Web 02/06/2007 Journal of Chemical and Engineering Data, Vol. 52, No. 2, 2007 581 -3 16 28 µ ) (0.9998 ( 0.0003); 4-PyCOOCH3, F)1.161 g‚cm , µ standard molar enthalpies of sublimation of naphthalene and ) (0.9980 ( 0.0005). of vaporization of n-decane.29 The determination of the enthalpy of sublimation of the 3-PyCOOCH3 isomer has not been Calorimetric Measurements performed using Calvet microcalorimetry because its low temperature of fusion and high vapor pressure did not allow Static Bomb Calorimetry. The energies of combustion of the the accurate use of this technique. three compounds were determined using an isoperibol static bomb calorimeter, with a twin valve bomb of internal volume Differential Scanning Calorimetry. Enthalpies and temper- 0.290 dm3. The calorimeter and technique have been previously atures of fusion of the three isomers were determined with a described.17-19 The energy equivalent of the calorimeter was Setaram DSC 141 calorimeter. Thermograms of samples determined twice during this experimental study, by the hermetically sealed in steel pans were recorded in an air · -1 combustion in oxygen of thermochemical standard benzoic acid atmosphere at a scanning rate of 0.017 K s . All the pans were weighed before and after the experiments to confirm that no NBS, SRM 39j, with the massic energy of combustion of ∆cu )-(26434 ( 3) J‚g-1 (value under certificate conditions). The product leakage had occurred. A Mettler UMT2 microbalance, × -7 calibration results were corrected to give the energy equivalent with a sensitivity of (1 10 ) g, has been used. (cal) corresponding to the average mass of water added to the For each compound, at least five samples weighing (8 to 10) calorimeter, 2900.0 g. For the study of isomers 3-PyCOOCH3 mg were recorded. To verify the stability of each compound and 4-PyCOOCH3, one set of 12 calibration experiments was after melting, several scans were performed for each sample. made in oxygen at p ) 3.04 MPa with 1.00 cm3 of water added Both the cooling and heating scans were reproducible after -1 to the bomb and yielded cal ) (15546.5 ( 1.1) J‚K , where several cycles for the three isomers. The samples were studied the uncertainty quoted is the standard deviation of the mean. over the following temperature ranges: (253 to 303) K for For the study of 2-PyCOOCH3, another set of 11 calibration 2-PyCOOCH3, (293 to 323) K for 3-PyCOOCH3, and (263 to -1 - experiments was made, yielding cal ) (15553.3 ( 0.9) J‚K . 303) K for 4-PyCOOCH3. No solid solid-phase transitions were For the combustion experiments, crystalline 3-PyCOOCH3 was observed for the three isomers over these temperature intervals. burnt in pellet form whereas the liquids 2-PyCOOCH3 and The cooling scans from above the melting temperature of 4-PyCOOCH3 were enclosed in melinex bags, according to the each compound toward the lower temperature limits were technique described by Skinner and Snelson.20 The massic achieved using circulating water from a thermostatic bath for ° energy of combustion of dry melinex, ∆cu )-(22902 ( 5) the study of 3-PyCOOCH3 and using circulating dry air, -1 20 J‚g , was confirmed by combustion of melinex samples in refrigerated with liquid nitrogen, for the study of 2-PyCOOCH3 our laboratory. The mass of melinex used in each experiment and of 4-PyCOOCH3. In both cases, an approximately constant was corrected for the mass fraction of water (0.0032).20 All the cooling rate of 0.017 K·s-1 was attained. Each melting curve samples were ignited at T ) (298.150 ( 0.001) K in oxygen at allowed computing the temperature and enthalpy of fusion of p ) 3.04 MPa, with 1.00 cm3 of deionized water previously the samples. The relative atomic masses used throughout this added to the bomb. paper were those recommended by the IUPAC Commission.30 The electrical energy for ignition ∆U(ign.) was determined Vapor Pressure Measurements. The vapor pressures of the from the change in potential difference across a 1400 µF three isomers were measured at different temperatures using a capacitor when discharged through the platinum ignition wire. static apparatus based on a capacitance diaphragm gage. The For the cotton thread fuse, of empirical formula CH1.686O0.843, apparatus and the measuring procedure have been recently -1 21 31 the massic energy of combustion is ∆cu° )-16250 J‚g . described in detail, so only a short description is given here. The corrections for nitric acid formation, ∆U(HNO3), were It is constructed of stainless steel tubing of internal diameter based on the value -59.7 kJ‚mol-1,22 for the molar energy of 17 mm with connections ConFlat DN 16 CF and includes all -3 formation of 0.1 mol‚dm HNO3(aq) from N2,O2, and H2O- metal angle valves, VAT series 57 high-temperature range for (l).
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